One of the most frequently asked technical
questions we get at VP is about the difference
between Motor, Research and R+M/2 octane
numbers. The next question often asked is why do
some fuel companies represent their fuels with
Motor Octane Numbers and other companies use
Research or R+M/2 Octane Numbers.

To answer these questions, we need to first
explain the machines that do the testing. These
machines, both the Motor Octane Machine and the
Research Octane Machine, were made in the 1930's
and were designed to test for octane numbers from
the 0-100 range. Any number above 100 is an
extrapolation. Both of these machines are dinosaurs
and are not adequate for today's high technology
fuels or engines, but they are all there is for testing
the octane of fuels.

These machines are one-cylinder engines that
have an adjustable head on them that can move up
or down to increase or lower the compression ratio
while the engine is running. The Motor and
Research machines are the same in this respect, but
they differ in several other characteristics. The
following is a comparison of the two machines used
for testing octane numbers:

As the comparison above shows, the Motor
Octane Machine is run at a higher RPM, hotter
temperature and with more timing. The Motor
Octane Machine will put more stress on any fuel
and more accurately represents a racing engine.
VP Racing Fuels always used the Motor Octane
Number when advertising our fuel or in our VP
Tech Bulletins because our fuels are primarily used
for racing applications.

The Research Octane Machine will always
produce a higher number for the obvious reason
that it does not put the same amount of stress on the
fuel sample. This number is sometimes used by
some fuel companies to trick the racer into thinking
he or she is getting a better fuel. The R+M/2 Octane
Number is the average of the Research and Motor
Octane numbers and is displayed on the retail level
gas pumps.

When comparing fuels for racing purposes make
sure to compare Motor Octane Numbers because
these are the ones that count in your racing
applications.